1. Field of the Invention
Embodiments of the present invention relate to electronic circuits and power supplies. More specifically, embodiments of the present invention relate to techniques for reducing acoustic noise from power supply inductors.
2. Related Art
Many modern computer systems operate under strict power consumption limitations. In order to meet these limitations, some computer systems support one or more low-power modes in which some of the computer's components operate using less power or are disabled. For example, during a low-power power mode, the computer system's hard drives can be stopped, the display can be deactivated, and/or the CPU clock can be slowed down.
When operating in full-power mode, the computer system draws current from a power supply according to the load on the computer system. For example, FIG. 1A presents a graph illustrating a current-flow pattern during full-power mode. As can be seen from FIG. 1A, the computer system draws different levels of power for different loads. For example, when a user loads a program from disk into memory, the disk, the memory, and the CPU are all activated. Hence, power usage increases, which increases the current drawn from the power supply. When the computer system subsequently finishes loading the program from disk, there is a corresponding decrease in the current drawn from the power supply.
When operating in low-power mode, the computer system can disable all but a minimal subset of computer system components. For example, the low-power mode may be a “sleep mode,” wherein all components are deactivated except a hardware monitor that is designed to wake the computer system upon receiving a communication from a peripheral (e.g., a keystroke or mouse movement). During the low-power mode, the computer system draws a small fraction of the power drawn during full-power mode. For example, FIG. 1B presents a graph illustrating a current-flow pattern during a low-power mode.
Some computer systems support a hybrid mode, which limits power consumption by dynamically disabling and re-enabling computer system components and features as they are used. Although the components and features are sometimes disabled in the hybrid mode, the computer system appears to be fully functional. For example, in some hybrid modes, the computer system may slow down the CPU clock when a user is not performing operations that require the full CPU power. In some systems, during the hybrid mode the computer system cycles between a specialized low-power mode and full-power mode at every opportunity (e.g., between keystrokes). For example, FIG. 1C presents a graph illustrating a current-flow pattern during a hybrid low-power/full-power mode. As shown in FIG. 1C, the computer system is subject to significant current swings during the hybrid mode (i.e., high di/dt).
Unfortunately, some computer systems include parts in the power supply that perform inadequately during such hybrid modes. For example, in some power supplies, an inductor will produce a clearly audible whine caused by the high di/dt when cycling back and forth between low-power mode and full-power mode. Because there are often limitations on the noise that computer systems (particularly laptops) may emit, an audible whine from the power supply may be unacceptable.
Hence, what is needed is a power supply for a computer system without the above-described problem.